Many different heating pad types are known in the art. One kind of heating pad generates heat with a chemical mixture operating in an exothermic reaction. Many such heaters have been described starting with the heating pad of Allison as disclosed in his 1907 U.S. Pat. No. 858,848. Other exothermic heating pads are described in such patents as to Mendez U.S. Pat. No. 2,612,155; Smith U.S. Pat. No. 1,620,581; Meagher U.S. Pat. No. 1,899,286; Baysinger U.S. Pat. No. 1,819,807; Glasser U.S. Pat. No. 3,301,250, and many others.
One version of exothermic heating pads involves a multiple compartment for the respective storage of chemicals and water. Activation of such device is obtained by squeezing to rupture a compartment (Foster U.S. Pat. No. 2,157,169) and cause an intermingling of the chemicals and water. Another multiple compartment exothermic heater or refrigerating device is described in Spencer U.S. Pat. No. 3,542,032.
The multiple compartment heating pad structure tends to be complex and creates the probability of premature rupture of the frangible partition, thus resulting in a premature activation of the heating pad. The overall reliability of such heating pad is, therefore, probably, at best, marginal.
Heating pads have also been extensively employed in the hair curling art. See, for instance, the patent to Sartory U.S. Pat. No. 1,565,510 wherein an exothermic heater is described for generating sufficient moist heat to set a curl in hair. The device includes a porous envelope of cheesecloth or paper or the like with an inner annular layer of chemical material to react with the water to produce heat. The hair curler exothermic heaters disclosed in the art propose a wide variety of chemicals generally selected to generate sufficient heat to establish hair setting temperatures.
Another exothermic heater is disclosed in the patent to Markel et al U.S. Pat. No. 2,153,671. In this patent an aluminum foil is held between two sheets of absorbent asbestos paper and held together with suitable staples. An external impervious material such as aluminum or tin foil is coated with an inner layer of waxed or parchment paper and wrapped around the aluminum inner layer with an inner sheet of felt material to form a heating pad. A suitable chemical material is added to react with the inner located aluminum foil and generate heat.
As is typical in exothermic heaters as described in Simmons U.S. Pat. No. 1,953,513 or Mendez U.S. Pat. No. 2,612,155, the opening which permits the ingress of water or aqueous solution must be so designed to prevent the potentially toxic reacting mixture from reaching the body surface of the user. Generally, complex water enclosures are employed to prevent injury to the user.
Another difficulty recognized with exothermic heating pads involves control over the reaction. Glasser U.S. Pat. No. 3,301,250 acknowledges this and teaches the use of a plastic bag to limit the access of air to the reaction site. The opening into the plastic bag is varied to regulate the reaction speed. The plastic bag further serves to retain moisture whose evaporation would result in a termination of the heat producing reaction.
Other moist heat generators found in the art are of the electrolytic type of which the patent to Durham et al U.S. Pat. No. 2,014,246 is typical. In Durham a permanent hair wave forming apparatus is described wherein hair setting moist heat is generated by applying an external electrical power source across an electrolyte to cause it to heat and generate steam. The steam is permitted to contact and set a hair curl. The application of power to such electrolytic heater involves a hazardous device which is, furthermore, cumbersome to use.
Uniform and thorough mixing of dry chemical mixtures when water or some other aqueous reacting medium is added in an exothermic heating pad is not always obtained, thus resulting in the generation of so-called "hot spots" lacking uniform heating of the structure. Also, the chemical mixture often has a tendancey to cake, thus severely limiting the flexibility of the total heat pad structure and preventing its conformance to the surface which is to be treated.
Another moist heating pad is taught by the patent to Jensen U.S. Pat. No. 2,710,008 wherein a flexible water pervious envelope is described. The envelope contains a hot water retaining bentonite clay of the montmorillonite type. Hot water is introduced into the bentonite filler by immersing the entire structure into boiling water for an extended time period. Upon removal of the structure from the boiling water, moist heat is generated through radiative cooling. The Jensen structure essentially acts as a heat sink and has a bulky cumbersome size and is not a self-contained heat source.
The Jensen heating pad depends upon the availability of a supply of boiling water or other suitable heat source. In addition, such pad is elaborate to employ and needs a substantial amount of time before heat can be applied. The user of the Jensen pad is generally completely immobilized and must employ substantial protective guards such as multiple towel wraps to prevent burning of body surfaces as a result of the boiling point temperature of the bentonite clay.
Another type of heat source is described in the patent to Kober U.S. Pat. No. 3,774,589 assigned to the same assignee as of this application. In the Kober structure an electrochemical cell is provided with discrete electrically located shorts to generate internal heating of the cell when it is activated by the addition of water. The electrochemical cell structure is of a non-toxic design and employs a safe electrolyte to enable one to employ such structure as a heating pad for the human body.